There are many situations in which it is necessary to separate a mixed granular or particulate material into granules or particles of different sizes, specific gravities or both. One example, in connection with which the present invention finds particular utility, is the environmental remediation of outdoor firearm training facilities which have become contaminated with lead from used bullets. In order to restore these sites to an uncontaminated condition, the lead bullets must be removed from the soil and rocks with which they are mixed, and the soil and rocks must then be chemically treated to stabilize any lead which has leached from the bullets before being returned to the site. Mechanical screening can, at least to some degree, be used to separate the mixture of soil, rocks and bullets into its component parts; however, since mechanical screening relies on size differences between the granules or particles to be separated, it is not capable of separating rocks and bullets which are of the same or similar size. Such separation is necessary to allow for recycling of the lead (which requires a certain level of purity in the product to be recycled) and to avoid having to remove a larger volume of material from the site than is necessary. By separating the lead bullets from similarly sized rocks, the rocks can be returned to the site after being chemically treated and the lead bullets can be removed from the site in a relatively pure form for recycling and reuse.
Air separation (also known as dry separation) provides a method for separating mixed granular or particulate materials into their component parts by relying on differences in the specific gravity (rather than size) of the granules or particles to be separated. The theory of air separation is well understood, and is described, for example, in U.S. Pat. Nos. 2,828,011, 4,519,896 and 5,032,256, the disclosures of which are expressly incorporated herein by reference. Briefly, air separation is carried out by allowing the mixed granular or particulate material to fall vertically by gravity across a horizontal stream or flow of air. Assuming that all of the granules or particles are of approximately the same size (and hence experience approximately the same drag force from the moving air), granules or particles of greater mass will be accelerated move slowly by the moving air than those of lesser mass. As a result, the heavier granules or particles will fall closer to the initial drop point than the lighter granules or particles. By positioning hoppers or receptacles at these locations, the heavier and lighter granules or particles can be collected and processed separately. Additional examples of air separators may be found in U.S. Pat. Nos. 775,965 and 2,978,103.
In theory, air separation provides a useful way to separate lead bullets from rocks of similar size in an environmental remediation operation of the type described above. In reality, however, there are a number of problems with this approach. For example, air separators are generally designed to operate with dry, easily separated granular or particulate materials, but the soil at an outdoor remediation site may be clumped or agglomerated as a result of precipitation, high clay content or other factors. This can result in poor separation between the rocks and lead bullets, in soil adhesion to both the rocks and lead bullets, and in clogging of the internal passages of the air separator. Another problem is the difficulty of adapting the air separator (whose geometry is generally fixed) to operate with granular or particulate materials other than those for which it was designed. In the case of an outdoor firing range, for example, the rocks found at different sites may have different specific gravities relative to that of lead; similarly, the lead to be removed from the site may in some cases consist of lead shot, which is relatively small in size, rather than lead bullets. In these situations, the use of an air separation process is practical only if the process can be adapted to the specific conditions encountered at the site.
Still another problem with existing types of air separators is the fact that the placement of the output hoppers or collection receptacles is dictated, at least to some extent, by the trajectories of the falling granular or particulate materials being separated. In air separators whose vertical dimensions or air flow rates are not large, the points at which the heavier and lighter granules or particles arrive at the bottom of the separator may be spaced apart by a relatively small horizontal distance. If hoppers or chutes are placed at these points and are arranged to discharge the separated granular materials vertically downward from the bottom of the separator, the discharge locations will also be relatively close together. This can be disadvantageous if, for example, the separated granular or particulate materials are to be discharged onto powered conveyors whose dimensions require that a certain minimum separation be maintained between them. It is possible to increase the distance between the discharge locations by angling the hoppers or chutes away from each other, but this results in discharge paths for the granular or particulate material that are more nearly horizontal and hence more prone to clogging, particularly if the granular or particulate material is wet or moist.